Listeriosis

Key messages

Listeriosis is an ‘urgent’ notifiable condition that must be notified immediately to the department by medical practitioners and pathology services. Laboratories are required to notify Listeria monocytogenes isolated from food or water.

L. monocytogenes is widespread in the environment and commonly isolated from sewage, silage, sludge, birds, and wild and domestic animals.

People in high-risk groups for listeriosis should avoid certain high-risk foods.

Notification requirement for listeriosis

Listeriosis is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days.

Laboratories are required to notify Listeria monocytogenes isolated from food or water.

These are Victorian statutory requirements.

Primary school and children’s services centre exclusion for listeriosis

Exclusion is not required.

Infectious agent of listeriosis

L. monocytogenes is a Gram-positive rod bacterium belonging to the genus Listeria. Of the seven recognised species, it is currently the only one implicated in human cases. There are at least 13 serovars of L. monocytogenes.

Identification of listeriosis

Clinical features

Listeriosis predominantly affects:

people who have immunocompromising illnesses such as leukaemia, diabetes or cancer

the elderly

pregnant women and their fetuses

newborn babies

people on immunosuppressive drugs such as prednisolone or cortisone.

Healthy adults are usually not affected but may experience transient, mild to moderate flu-like symptoms. Ingestion of contaminated food by healthy, nonpregnant individuals can produce a self-limited febrile gastroenteritis.

Infection in pregnant women may be mild, and a temperature before or during birth may be the only sign. However, the infection can be transmitted to the fetus through the placenta, which can result in stillbirth or premature birth. Babies may be severely affected with conditions such as bacteraemia or meningitis (early-onset neonatal listeriosis).

Late-onset neonatal listeriosis generally affects full-term babies who are usually healthy at birth. The onset of symptoms in these babies occurs several days to weeks after birth (a mean of 14 days), possibly as a result of infection acquired from the mother’s genital tract during delivery or postnatally through cross-infection.

In nonpregnant immunosuppressed patients, listeriosis usually presents as an acute meningoencephalitis, brain stem encephalitis (rhomboencephalitis), brain abscess or bacteraemia. Focal infections such as pneumonia, endocarditis, infected prosthetic joints, localised internal abscesses, and granulomatous lesions in the liver and other organs have also been described. Symptoms may have a sudden onset; however, in 60 per cent of cases, presentation is subacute (>24 hours). Fever, severe headache, nausea and vomiting can lead to prostration and shock.

The reported case-fatality rate has been around 30 per cent in both pregnancy- and nonpregnancy-related groups.

Incubation period of Listeria monocytogenes

The incubation period is mostly unknown. Outbreak cases have occurred 3–70 (mean 31) days after a single exposure to an implicated product. Median incubation is estimated to be 3 weeks.

Public health significance and occurrence of listeriosis

Listeriosis is an uncommon disease in humans. In Australia in 2013, the rate was three infections per million population.

Reservoir of Listeria monocytogenes

L. monocytogenes is widespread in the environment and commonly isolated from sewage, silage, sludge, birds, and wild and domestic animals. It has caused infection in many animals, and resulted in abortion in sheep and cattle. The bacteria are commonly isolated from poultry. It is a common contaminant of raw food.

Asymptomatic vaginal carriage occurs in humans, and faecal carriage of up to 5 per cent in the general population has been reported. The significance of these carriers in the epidemiology of listeriosis is unknown.

Mode of transmission of Listeria monocytogenes

The main route of transmission is oral, through ingestion of contaminated food. Other routes include mother-to-fetus transmission via the placenta or at birth. The infectious dose is unknown. Horizontal human-to-human infection has not been documented.

Period of communicability of listeriosis

Mothers of infected newborns may shed the infectious agent in vaginal discharges and urine for 7–10 days after delivery. Infected individuals can shed the organisms in their stools for several months.

Susceptibility and resistance to listeriosis

Although healthy people can be infected, the disease generally affects vulnerable groups in the community, such as:

people who have immunocompromising illnesses (such as leukaemia, diabetes, cancer)

the elderly

pregnant women and their fetuses

newborn babies

people on immunosuppressive drugs (such as prednisolone or cortisone).

There is little evidence of acquired immunity, even after prolonged severe infection.

Control measures for listeriosis

Preventive measures

It is important to educate people in high-risk groups about the foods likely to be contaminated, and about safe food handling and storage.

People in high-risk groups for listeriosis should avoid the following high-risk foods:

ready-to-eat seafood such as smoked fish and mussels, oysters, and raw seafood such as sashimi or sushi

pre-prepared or stored salads, including coleslaw and fresh fruit salad

drinks made from fresh fruit or vegetables where washing procedures are unknown (excluding canned or pasteurised juices)

precooked meat products that are eaten without further cooking or heating, such as pate, sliced deli meat (including ham, strasburg and salami) and cooked diced chicken (as used in sandwich shops)

any unpasteurised milk or foods made from unpasteurised milk

soft-serve ice-creams

soft cheeses, such as brie, camembert, ricotta and feta (these are safe if cooked and served hot)

ready-to-eat foods, including leftover meats that have been refrigerated for more than 1 day

dips and salad dressings in which vegetables may have been dipped

raw vegetable garnishes.

Safe foods include:

freshly prepared foods

freshly cooked foods, to be eaten immediately

hard cheeses, cheese spreads and processed cheese

milk – freshly pasteurised and UHT

yoghurt

canned and pickled food.

Safe food handling and storage advice includes the following:

Wash your hands before preparing food, and between handling raw and ready-to-eat foods.

Keep all food covered during storage.

Place all cooked food in the refrigerator within 1 hour of cooking.

Store raw meat, raw poultry and raw fish on the lowest shelves of your refrigerator to prevent them from dripping onto cooked and ready-to-eat foods.

Keep your refrigerator clean and the temperature below 5 °C.

Strictly observe use-by and best-before dates on refrigerated foods.

Do not handle cooked foods with the same utensils (tongs, knives, cutting boards) used for raw foods, unless they have been thoroughly washed with hot soapy water between uses.

All raw vegetables, salads and fruits should be well washed before eating or juicing, and consumed fresh.

Defrost food by placing it on the lower shelves of a refrigerator or using a microwave oven.

Reheat food until the internal temperature of the food reaches at least 70 °C (piping hot).

Reheat leftovers until piping hot.

When using a microwave oven, read the manufacturer’s instructions carefully and observe the recommended standing times, to ensure that the food attains an even temperature before it is eaten.

Foods are regularly tested for the presence of L. monocytogenes. Processed, packaged ready-to-eat foods found to be contaminated with L. monocytogenes are recalled from sale.

Control of case

Treatment is usually with benzylpenicillin either alone or in combination with trimethoprim + sulfamethoxazole. For penicillin-sensitive patients, trimethoprim + sulfamethoxazole may be used alone (see the current edition of Therapeutic guidelines: antibiotic). Carbapenems are an alternative. Duration of therapy depends on immune and pregnancy status of the patient; it generally ranges from 2 weeks for uncomplicated bacteraemia to 6 weeks for rhomboencephalitis or brain abscess. Treatment should be guided by an infectious diseases specialist.

Outbreak measures for listeriosis

Obtain a medical history from the treating doctor.

Obtain a food history from the patient.

Test any available suspected foods.

Assess the possibility of common-source outbreaks if there is a cluster of cases.

Epidemiological investigation of cases should be used to detect outbreaks and to determine source.

Molecular subtyping should be used to determine the association between isolates from cases and any foods positive for L. monocytogenes.

Investigate the source of any foods found to be positive for L. monocytogenes to determine the point at which they became contaminated.

Recall contaminated food, if necessary.

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